Wireless communication frame design for medical implants

ABSTRACT

Techniques provided herein are directed toward providing a robust downlink communication frame that enables medical implants with highly inaccurate LOs to reliably provide uplink communications to an interrogator device. The downlink communication frame can include, among other things, a plurality of uplink trigger subframes that enable timing of uplink communication of the various medical implants with which the interrogator device is communicating. These uplink trigger subframes may be modulated in a special manner as to distinguish them from other subframes.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/487,443, filed Apr. 19, 2017, entitled “WIRELESS COMMUNICATION FRAMEDESIGN FOR MEDICAL IMPLANTS”, which is assigned to the assignee hereof,and incorporated by reference herein in its entirety.

BACKGROUND

A wireless medical implant system for a patient can comprise aninterrogator device, typically in, on, or in proximity to the patient,and a plurality of electronic medical implants that can take biologicalmeasurements of a body part (e.g., biological tissue) and communicatethem to the interrogator device. The interrogator device can thencommunicate this information to other devices, such as a mobile phone,tablet, or medical device of the patient or patient's healthcareprovider. The interrogator device can also communicate with the medicalimplants to cause them to stimulate the body part.

However, the medical implants may need to operate on very low powerconsumption. This can provide severe power constraints on the design ofthe local oscillator used for wireless communication. Thus, to preservepower in a medical implant system, wireless communication may need totolerate relatively large inaccuracies in the carrier frequency. Inaddition, these medical implants may be very small and may not havephysical space to include a crystal, for a crystal oscillator, which isanother reason the local oscillator frequency may be relativelyinaccurate.

SUMMARY

Techniques provided herein are directed toward providing a robustdownlink communication frame that enables medical implants with highlyinaccurate LOs to reliably provide uplink communications to aninterrogator device. The downlink communication frame can include, amongother things, a plurality of uplink trigger subframes (or fields) thatenable timing of uplink communication of the various medical implantswith which the interrogator device is communicating. These uplinktrigger subframes may be modulated in a special manner as to distinguishthem from other subframes.

A medical device, according to the disclosure, comprises a communicationinterface configured to receive a communication frame transmitted via aradio frequency (RF) signal and comprising a synchronization subframe, apayload subframe, and a plurality of uplink trigger subframes spacedapart such that at least one uplink subframe can be transmitted betweensuccessive uplink trigger subframes. The medical device furthercomprises a processing unit communicatively coupled with thecommunication interface and configured to determine when to send anuplink subframe via the communication interface based, at least in part,on when at least one of the plurality of uplink trigger subframes wasreceived, and send, via the communication interface, the uplinksubframe.

The medical device can comprise one or more the following features. Theprocessing unit may be configured to determine when to send the uplinksubframe by counting a number of uplink trigger subframes of theplurality of uplink trigger subframes, by obtaining an identifier fromthe at least one of the plurality of uplink trigger subframes, or byadjusting the local clock based on when the at least one of theplurality of uplink trigger subframes was received. The processing unitmay be further configured to identify the at least one of the pluralityof uplink trigger subframes by analyzing a pair of pulses in the atleast one of the plurality of uplink trigger subframes, wherein the pairof pulses is modulated such that a first pulse has a first duration, anda second pulse has a second duration different than the first duration.The processing unit may be configured to analyze the pair of pulses inthe at least one of the plurality of uplink trigger subframes bycomparing a ratio of the first duration to the second duration. Theprocessing unit may be configured to send the uplink subframe betweensuccessive uplink trigger subframes of the plurality of uplink triggersubframes.

An interrogator device, according to the description, comprises aprocessing unit configured to generate a communication frame comprisinga synchronization subframe, a payload subframe, and a plurality ofuplink trigger subframes. The interrogator device further comprises acommunication interface communicatively coupled with the processing unitand configured to send the communication frame via a radio frequency(RF) signal, and receive at least one uplink frame between successiveuplink trigger subframes in the communication frame.

The interrogator device may comprise one or more of the followingfeatures. The processing unit may be further configured to include, ineach uplink trigger subframe of the plurality of uplink triggersubframes, an identifier unique to the respective uplink triggersubframe. The processing unit may be further configured to include ineach uplink trigger subframe of the plurality of uplink trigger subframea first pair of pulses, wherein the first pair of pulses uses a firstmodulation scheme such that a first pulse has a first duration, and asecond pulse has a second duration different than the first duration.The processing unit may be further configured to cause a ratio of thefirst duration to the second duration to exceed a threshold. Theprocessing unit may be configured to use a second modulation scheme tomodulate a second pair of pulses in the payload subframe, wherein thesecond modulation scheme is different than the first modulation scheme.

An example method of synchronizing wireless communication at a medicaldevice, according to the description, comprises receiving, at themedical device, a communication frame transmitted via a radio frequency(RF) signal and comprising a synchronization subframe, a payloadsubframe, and a plurality of uplink trigger subframes spaced apart suchthat at least one uplink subframe can be transmitted between successiveuplink trigger subframes. The method further comprises determining, withthe medical device, when to send an uplink subframe based, at least inpart, on when at least one of the plurality of uplink trigger subframeswas received, and sending, with the medical device, the uplink subframe.

The method can further comprise one or more of the following features.Determining when to send the uplink subframe may comprise counting anumber of uplink trigger subframes of the plurality of uplink triggersubframes, obtaining an identifier from the at least one of theplurality of uplink trigger subframes, or adjusting a local clock of themedical device based on when the at least one of the plurality of uplinktrigger subframes was received. The method may further compriseidentifying the at least one of the plurality of uplink triggersubframes by analyzing a pair of pulses in the at least one of theplurality of uplink trigger subframes, wherein the pair of pulses ismodulated such that a first pulse has a first duration, and a secondpulse has a second duration different than the first duration. Analyzingthe pair of pulses in the at least one of the plurality of uplinktrigger subframes comprises comparing a ratio of the first duration tothe second duration. Sending the uplink subframe may comprise sendingthe uplink subframe between successive uplink trigger subframes of theplurality of uplink trigger subframes.

A method of enabling synchronized wireless communication with ainterrogator device, according to the description, comprises generating,with the interrogator device, a communication frame comprising asynchronization subframe, a payload subframe, and a plurality of uplinktrigger subframes. The method further comprises sending, with theinterrogator device, the communication frame via a radio frequency (RF)signal, and receiving, at the interrogator device, at least one uplinkframe between successive uplink trigger subframes in the communicationframe.

The method may further comprise one or more of the following features.The method may comprise including, in each uplink trigger subframe ofthe plurality of uplink trigger subframes, an identifier unique to therespective uplink trigger subframe. The method may comprise including ineach uplink trigger subframe of the plurality of uplink trigger subframea first pair of pulses, wherein the first pair of pulses uses a firstmodulation scheme such that a first pulse has a first duration, and asecond pulse has a second duration different than the first duration.The method may comprise causing a ratio of the first duration to thesecond duration to exceed a threshold. The first modulation scheme maybe different than a second modulation scheme used to modulate a secondpair of pulses in the payload subframe.

BRIEF DESCRIPTION OF DRAWINGS

Non-limiting and non-exhaustive aspects are described with reference tothe following figures, wherein like reference numerals refer to likeparts throughout the various figures unless otherwise specified.

FIG. 1 is a simplified cross-sectional diagram illustrating anembodiment of a wireless medical implant system.

FIG. 2A is an illustration of how calibration frames and can becommunicated and what they may comprise, according to some embodiments.

FIG. 2B is a diagram of a pulse pair that illustrates how signalscarrying the calibration frames may be modulated, according to anembodiment.

FIG. 3 is an illustration of how operating frames can be communicatedand what they may comprise, according to some embodiments.

FIG. 4 is an illustration of example pulse pair that can be used in anuplink trigger subframe.

FIG. 5 is an illustration of example contents of an uplink subframe,according to an embodiment.

FIG. 6 is a flow diagram of an example method of synchronizing wirelesscommunication at a medical device, according to an embodiment.

FIG. 7 is a flow diagram of an example method of enabling synchronizewireless communication with an interrogator device, according to anembodiment.

FIG. 8 is a simplified block diagram of an interrogator device,according to an embodiment.

FIG. 9 is a simplified block diagram of a medical implant, according toan embodiment.

Elements, stages, steps, and actions in the figures with the samereference label in different drawings may correspond to one another(e.g., may be similar or identical to one another). Further, someelements in the various drawings are labelled using a numeric prefixfollowed by a numeric suffix (where the numeric prefix and the numericsuffix are separated by a hyphen). Elements with the same numeric prefixbut different suffices may be different instances of the same type ofelement. The numeric prefix without any suffix is used herein toreference any element with this numeric prefix.

DETAILED DESCRIPTION

Several illustrative embodiments will now be described with respect tothe accompanying drawings, which form a part hereof. The ensuingdescription provides embodiment(s) only, and is not intended to limitthe scope, applicability or configuration of the disclosure. Rather, theensuing description of the embodiment(s) will provide those skilled inthe art with an enabling description for implementing an embodiment. Itis understood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope ofthis disclosure.

It will be understood by a person of ordinary skill in the art that,although the embodiments provided herein are directed toward medicalapplications, the techniques described herein may be utilized in otherapplications involving digital communication. Additionally, embodimentsprovided herein describe the use of “medical implants,” although suchimplants may be utilized to gather data and/or stimulate a body partwithout necessarily performing a medical function. Moreover, andindicated below, embodiments may utilize medical devices that may or maynot be partially or wholly implanted or implantable. A person ofordinary skill in the art will recognize many variations.

FIG. 1 is a simplified cross-sectional diagram illustrating anembodiment of a wireless medical implant system. Here, a patient's head110 is illustrated, indicating a portion of the brain 120 in which aplurality of medical implants 130 are implanted. (For clarity, only aportion of the medical implants are labeled.) An interrogator device 140uses low-power, short-range radio frequency (RF) signals at a designatedfrequency not only to communicate with the one or more medical implants,but also, in some embodiments, to provide power to the implants. Suchwireless communication can employ any of a variety of short-rangewireless technologies, including near-field communication (NFC) and/orother wireless technologies. According to some embodiments, data may becommunicated in a secure fashion (e.g., using any of a variety ofencryption techniques).

For scenarios in which the wireless medical implant system is utilizedto measure and stimulate a portion of the brain (as shown in FIG. 1),the interrogator device 140 may be referred to as a “skin patch” becauseit may be substantially flat in shape and may be disposed on or near thepatient's skin. The medical implants 130 in such scenarios may bereferred to as “neurograins” because of their relatively small size andlocation within the patient's brain. A person of ordinary skill in theart will appreciate that alternative embodiments may have medicalimplants located in one or more other areas (other than the brain) of apatient's body and/or in or other wireless devices non-medical wirelesscommunication applications. Moreover, for medical applications,interrogator device 140 may comprise one or more physical units locatedin, on and/or in proximity to the patient's body.

Depending on the application, the wireless medical implant system maycomprise hundreds or thousands of medical implants 130. (Alternativeembodiments may include a smaller or larger number of medical implants130 than this.) These medical implants 130 can also communicate back tothe interrogator device 140 (e.g., through RF backscatter, by changingthe impedance of their respective antennas) using, for example, a timedivision multiple access (TDMA) protocol. The interrogator device 140may coordinate the uplink transmission.

Medical implants 130 can comprise active devices (having a power source)and/or passive devices (having no power source) configured to takebiological measurements of the brain 120 (e.g., information regardingelectrical signals generated by the patient's brain cells) andcommunicate the measurements to the interrogator device 140 and/orprovide stimulation of the patient's brain 120 (e.g., via one or moreelectrodes), where such stimulation may be based on communicationreceived from the interrogator device 140. In some embodiments, activemedical implants may also draw power wirelessly from the interrogatordevice, which may be used to charge their batteries (or other powersources), or the implant may work directly off the wireless powerwithout having a battery. As previously noted, medical implants 130 canbe powered by the interrogator device 140 using, for example, a coiledantenna drawing power from communications and/or other signals or fieldsgenerated by the interrogator device 140. It can be noted that, inalternative embodiments, the interrogator device 140 may comprisemultiple antennas, and/or the biological measurement and stimulationsystem may have one or more nodes and/or devices between the medicalimplants 130 and the interrogator device 140. Because medical implants130 can vary in functionality, they can vary in size, shape, type,and/or may have electrodes (and or other sensors) that vary as well.

A person of ordinary skill in the art will appreciate the basic hardwareconfiguration of an interrogator device 140 and/or medical implant 130.This can include, for example, a power source, processing unit,communication bus, volatile and/or non-volatile memory (which maycomprise a non-transitory computer-readable medium having computer codefor execution by the processing unit), transceiver, antenna, etc. Themedical implant 130 may further comprise one or more sensors,electrodes, and/or stimulators utilized for sensing and/or stimulatingone or more parts of the body. As such, the interrogator device 140and/or medical implant 130 may have means for performing some, or all,of the functions described herein using one or more of its hardwareand/or software components. In some embodiments, components may beselected and/or optimized for low power consumption. In particular,because medical implants 130 may be limited in size and/or power, themedical implants 130 may not have the same memory size and/or processingcapabilities as the interrogator device 140. Example electrical hardwareand software components of an interrogator device 140 and medicalimplant 130 are illustrated in FIG. 8 and FIG. 9, respectively, anddescribed in more detail below.

As noted above, the medical implants 130 may be passive or activeimplants that collect energy from the interrogator device 140, andthereby may need to operate on very low power consumption. But this canprovide severe power constraints on the design of the local oscillator(LO) used for RF communication, because the accuracy of the LO isdirectly related to power consumption: low-power LOs are generally lessaccurate than relatively higher-power LOs. In addition, due to sizeconstraints there is insufficient area to include a crystal on themedical device, making it more difficult to produce an accurate LOfrequency. Thus, to preserve power in a medical implant system, wirelesscommunication may need to tolerate relatively large inaccuracies in thecarrier frequency. (Because an interrogator device is typically given alarger power budget than the medical implant, it may have a moreaccurate LO then the medical implant. The medical implant, however, mayhave a highly inaccurate LO.)

For example, in some embodiments, the LO of the medical implants (whichmay comprise a simple ring oscillator) can vary in frequency accuracyfrom approximately ±10% to approximately ±30%. Other embodiments mayexperience larger or smaller frequency inaccuracies. However, downlinkcommunications from the interrogator device to the medical implantsoccurs at a low data rate, using, for example, only three bits permedical implant per frame, though the number of bits transmitted to eachmedical implant may be more or less than three bits per frame

Traditional implementations of a wireless medical implant systemattempted to overcome issues that arise with inaccurate medical implantLOs by removing them altogether and instead deriving a local clock ofthe medical implant from two RF carriers transmitted by the interrogatordevice. Although this design facilitates synchronization, it results insignificant interference in wireless medical implant systems where manymedical implants are utilized.

To avoid this interference, embodiments provided herein may be utilizedby a wireless medical implant systems within interrogator device havinga single RF carrier and each medical implant having its own LO togenerate its local clock. Interference may be reduced, at least in part,since the LOs of the various medical implants will not be in phase, sointerference caused by non-transmitting operations performed by themedical implants is reduced, being spread over time. However, as notedpreviously, the LO of the medical implants can be highly inaccurate.

Techniques disclosed herein are directed toward providing a robustdownlink communication frame in a wireless medical implant system wheredownlink communication from an interrogator device of the wirelessmedical implant system to one or more medical implants may be impactedby the inaccuracy of the LO of the medical implants. In particular, thedownlink communication frame can include, according to embodiments, aplurality of uplink trigger subframes that enable timing of uplinkcommunication of the various medical implants with which theinterrogator device is communicating. These uplink trigger subframes maybe modulated in a special manner as to distinguish them from othersubframes.

The downlink communication frame having uplink trigger subframes maycomprise an operating frame, in which operating data is communicated toand/or from medical implants 130. Optionally, however, downlinkcommunication may additionally include a calibration frame used to helpcalibrate LOs of medical implants.

FIG. 2A is an illustration of how calibration frames 200-1 and 200-2(collectively and generically referred to herein as calibration frames200) can be communicated and what they may comprise, according to someembodiments. When the wireless medical implant system is first activatedand the medical implants are first powered on, the LO frequencies of thevarious medical implants may vary significantly, as noted above. Thesecalibration frames 200 provide one way in which the accuracy of the LOsmay be improved.

Generally speaking, a calibration frame 200 is a downlink frame (sentfrom the interrogator device to the medical implants) with signalsidentifiable by the medical implants as calibration signals, enablingthe medical implants to calibrate their LOs. Put briefly, thecalibration frame 200 may include only downlink information (withoutwaiting for an uplink response), and may be modulated differently toidentify it as a calibration frame. Additional details regardingcalibration are provided herein below as well as U.S. Pat. App. No.62/480,945 entitled “Pulse Width Modulated Amplitude Modulation” whichis hereby incorporated by reference in its entirety for all purposes.(This application referred to herein below as “the '945 application”.)This calibration can improve the accuracy of the LOs to approximately±200 parts per million (ppm). In alternative embodiments where lasertrimming or calibration is performed during the manufacturing of themedical implants (which may reduce the frequency variation in the LOs toless than 1%), a calibration frame may not be needed. As illustrated,calibration frames 200 may be repeated any number of times (depending ondesired functionality), if desired, to help ensure calibration of themedical implants.

As illustrated, the calibration frame comprises a frame sync subframe210 to allow medical implants to find the beginning of the calibrationframe 200. A downlink payload subframe 220 that (in the case of thecalibration frame 200) carries information indicating that it is acalibration frame. Following the downlink payload subframe 220, theremay be a relatively long period in which there is no downlink data (butthe RF carrier may still be transmitted by the interrogator device topower the medical implants).

FIG. 2B is a diagram of a pulse pair 230 that illustrates how signalscarrying the calibration frames 200 may be modulated, according to anembodiment. As detailed in the '945 application, a digital bit ofinformation may be encoded in a pair of pulses having differentamplitudes. For example, each bit may comprise a first pulse with a“high” amplitude and a second pulse with a “low” amplitude. (This is,however, chosen arbitrarily. Alternative embodiments may choose to havea “low” first pulse and a “high” second pulse. Additionally, theamplitude of the “high” and “low” pulses may vary, as detailed in the'945 application.) In the example, a frame sync subframe 210 an includethe pulse pair 230, which comprises a “high” pulse 240 many times longerthan the “low” pulse 250 that follows. In some embodiments, for example,the “high” pulse 240 may be six times longer than the “low” pulse 250.(Of course, the ratio between long and short pulses may be different inalternative embodiments.) Here, the length of the long “high” pulse inthe frame sync subframe 210 identifies the frame sync subframe as aframe sync subframe to the medical devices. Additional informationincluded in the calibration frame 200 (e.g., in the downlink payloadsubframe 220) may be encoded in a similar manner, where the value of adigital bit is based on which pulse in a pulse pair is longer (thefirst, “high” pulse or the second, “low” pulse), and where the ratiobetween long and short pulses is distinguishable over the pulses in theframe sync subframe (using, for example, a different ratio of pulselength between long and short pulses). Again, additional details areprovided in the '945 application.

FIG. 3 is an illustration of how operating frames 300-1 and 300-2(collectively and generically referred to herein as operating frames300) can be communicated and what they may comprise, according to someembodiments. As illustrated, operating frames 300 can include bothdownlink (interrogator device to medical implant) and uplink (medicalimplant to interrogator device) communications. Because the medicalimplants utilize backscatter, the interrogator device may send an RFcarrier during the entire operating frame, enabling the medical implantsto backscatter the RF carrier for uplink communications to theinterrogator device.

As illustrated, similar to the calibration frame 200 of FIG. 2A,operating frames 300 can include a frame sync subframe 310 and downlinkpayload 320. Here, the downlink payload 320 can include information forthe medical implants to, for example, control stimulation (turning iton/off, communicating a level of stimulation, etc.), brain functionsensing, and/or other functionality. Again, and data can be encoded inpulse pairs, as previously described.

In the operating frame, additional uplink trigger subframes 330-1,330-2, and 330-N (collectively and generically referred to herein asuplink trigger subframes 330) are also included, to help ensure themedical implants respond when scheduled. As illustrated in FIG. 3, thedownlink communication sent by the interrogator device includes theframe sync subframe 310, downlink payload subframe 320, and uplinktrigger subframes 330 followed by time slots for each of the medicalimplants to provide, in turn, uplink communications.

In a given operating frame, there may be a set number of slots, N, formedical implants to communicate. In some embodiments, for example, theremay be 1000 slots. Other embodiments may have a higher or lower numberof slots, depending on desired functionality. In some embodiments, thenumber of slots may be dynamic. The number N may be determined by theinterrogator device and conveyed to the medical implants in the downlinkpayload subframe 320, for example.

Because the LO of each medical implant may still vary (e.g., by ±200ppm) even after calibration, there is a chance that the medical implantsmay not be able to keep time accurately enough to communicate in theirdesignated slot if they perform local synchronization only once peroperating frame. For example, by the time slot number 1000 occurs in theoperating frame 300, the medical implant scheduled to communicate duringthat slot may have experienced enough clock drift from the beginning ofthe operating frame 300 to have caused it to have communicated already(perhaps interfering with another medical implants communications), orit may not communicate until after slot number 1000 occurs.

To help mitigate these types of errors, uplink trigger subframes 330 canbe located throughout the operating frame 300 to help keep the medicalimplants synchronized. As illustrated, some embodiments may include anuplink trigger subframe 330 before each slot for uplink communication.(In other words, for an operating frame 300 having N slots for uplinkcommunication, there will be N uplink trigger subframes 330.) Someembodiments may include an uplink trigger subframe before a group of twoor more slots for uplink communication. (E.g., for each uplink trigger agroup of medical implants can respond, one at a time, before the nextuplink trigger. In other words, for an operating frame 300 having Nslots for uplink communication, there may be M uplink trigger subframes330, where M=N/2, N/3, or some other fraction of N, depending on desiredfunctionality.). The use of uplink trigger subframes 330 thereforeenables each medical implant to more accurately determine the passage oftime during the operating frame 300 by identifying the uplink triggersubframes 330 (which are based on the LO of the interrogator device,which is far more accurate). In other words, medical devices cansynchronize with each uplink trigger subframe 330 to help mitigate theeffects of clock drift during the operation frame 300.

According to embodiments, the length of time between uplink triggerframes may be larger than the length of time for uplink communication,allowing for medical implants to respond to the uplink trigger subframe,and also allowing for clock error (expansion or compression of uplinktransmission due to clock inaccuracy). For example, in cases whereuplink trigger subframes are sent for each uplink slot (as illustrated),there may be a period of time, Δ₁, after the interrogator devicecompletes the transmission of an uplink trigger subframe 330-1 andbefore a medical implant begins transmission of a corresponding uplinksubframe 340-1 (labeled in FIG. 3 as “UL 1”). This period of time may beset based on a determined period of time it may take the interrogatordevice to change from a transmission mode (e.g., transmitting the uplinktrigger subframe 330-1) to a receive mode (e.g., ready to receive thecorresponding uplink subframe 340-1), and may allow for a certain (e.g.,expected maximum) amount of clock error of the medical devices.

There may also be a second period of time, Δ₂, after the medical implantends transmission of the corresponding uplink subframe 340-2 and beforethe interrogator device begins transmission of the subsequent uplinktrigger subframe 330-2. Due to clock drift during the transmission ofthe corresponding uplink subframe 340- 1, the value of Δ₂ may be subjectto more variance than the value of Δ₁. Accordingly, the interrogatordevice may take into account worst-case clock inaccuracies of themedical devices. That said, because clock error of the medical implantscan be reduced to less than 1% once calibrated, the additional overheadneeded to accommodate worst-case clock expansion remains minimal. Thevalue of Δ₂ may also take into account a length of time it may take theinterrogator device to change from a receive mode back to a transmissionmode.

Ultimately, the interrogator device may determine a period of time, T,between the transmittal of uplink trigger subframes 330 by adding amaximum expected value Δ₁, a length of time of an uplink subframe 340,and a maximum expected value of Δ₂. As previously noted, the maximumexpected value of Δ₂ can be based on worst-case clock expansion what theuplink trigger subframe 330, and therefore time T can be set toaccommodate different amounts of clock error. Depending on the amount ofclock error allowable, the calibration frame to reduce clock error maynot be needed, in some embodiments.

In some embodiments, the uplink trigger subframes 330 may be identical.In this case, each medical implant may utilize a counter that counts thenumber of uplink trigger subframes 330 in an operating frame to ensurethat it communicates in its designated uplink slot. For instance, whenthe value of the counter matches an address of the medical implant aftercounting the latest uplink trigger subframe 330, the medical implant maythen send an uplink subframe 340 during the time slot following theuplink trigger subframe 330.

In some embodiments, each uplink trigger subframe 330 may includeinformation such as a counter, medical device address, or otheridentifier to indicate its position in the operating frame 300 and/orindicate the medical device designated to provide an uplink subframe 340in the following uplink slot. In this case, the medical implant may notneed to include the additional hardware (and/or software) required toimplement a counter.

As indicated in FIG. 3 with a second operating frame 300-2 following afirst operating frame 300-1, operating frames 300 can repeat. In thesubsequent operating frame(s), new frame sync and downlink payloadsubframes can be transmitted and communications communicating with thesame medical implants or different medical implants as the firstoperating frame 300-1 can follow in a manner similar to that describedabove. (Where different operating frames 300 communicate with differentsets of medical implants, the downlink payload subframe 320 of theoperating frame 300 may indicate which subset of medical implants isselected for communication during the operating frame 300 with theinterrogator device.) The total length of the operating frame can vary,depending on latency, number of medical implants, and/or other factors.In some embodiments, for example, the operating frame is about 100 ms.(In other embodiments, the operating frame may be longer or shorter.)

Similar to the frame sync subframes in calibration and operations framesdescribed above, the uplink trigger subframe 340 can include digitalbits comprising pulse pairs modulated differently than pulse pairs inother communications. FIG. 4 is an illustration of example pulse pair410 that can be used in an uplink trigger subframe 340, to identify theuplink trigger subframe 340 as such. Similar to the frame syncsubframes, the pulse pair 410 includes a long pulse 420 and a shortpulse 430, where the ratio of the length of time of the long pulse 420to the length of time of the short pulse 430 is especially high (therebymaking it identifiable to medical devices). In this embodiment, however,the first (“high”) pulse is the short pulse 430, and the second (“low”)pulse is the long pulse 420, making the ordering of the short and longpulses opposite from that of the short and long pulses of the phrasesync subframes (as illustrated, for example, in FIG. 2B). Thus, althoughthe ratio of long to short pulses may be similar, the medical devicescan distinguish between the frame sync subframes and the uplink triggersubframes by determining the order of the long and short pulses in thepulse pairs.

FIG. 5 is an illustration of example contents of an uplink subframe 340,according to an embodiment. As previously indicated, each medical devicecan transmit and uplink subframe during its designated transmissionslot, which can include data specific to that medical device. Here, theuplink subframe 340 can comprise an uplink preamble 510 followed by anuplink payload 520. The uplink preamble 510 can be used by theinterrogator device to perform timing correction and, in someembodiments, may comprise a maximal length sequence. The uplink payload520 carries data (e.g., operating status, sensor data, etc.) from themedical device to the interrogator device.

FIG. 6 is a flow diagram of an example method 600 of synchronizingwireless communication at a medical device, according to an embodiment.Here, the functionality described in one or more of the blocksillustrated in FIG. 6 may be performed, for example, by a medical devicein a low-power wireless system, such as a medical implant 130 of thewireless medical device system illustrated in FIG. 1. Accordingly, meansfor performing this functionality may include hardware and/or softwarecomponents of a medical implant 130. An example of such hardware and/orsoftware components is illustrated in FIG. 9 and described in moredetail below. Nonetheless, it will be understood that medical devicesother than implanted or implantable medical devices may be utilized, insome embodiments.

The functionality at block 610 includes receiving, at the medicaldevice, a communication frame transmitted via an RF signal andcomprising a synchronization subframe, a payload subframe, and theplurality of uplink trigger subframes spaced apart such that at leastone uplink subframe can be transmitted between successive uplink triggersubframes. Here, the communication frame may comprise an operatingframe, as described in the embodiments above and illustrated in FIG. 3.As previously noted, the time between successive subframes (e.g., time Tin FIG. 3) can allow for a buffer of time before and after thetransmittal of an uplink subframe, and may accommodate expectedworst-case clock drift for medical devices. Means for performing thefunctionality at block 610 may comprise, for example, bus 905,processing unit(s) 910, memory 920, communication interface 930, antenna935, and/or other components of a medical implant 130, as illustrated inFIG. 9 and described in more detail below.

At block 620, the functionality comprises determining, with of themedical device, when to send an uplink subframe based, at least in part,on when at least one of the plurality of uplink trigger subframes wasreceived. In some embodiments, determining when to send the uplinksubframe may comprise counting a number of uplink trigger subframes ofthe plurality of uplink trigger subframes. In some embodiments, this maybe done by a dedicated hardware counter of the medical device.Additionally or alternatively, in some embodiments, determining when tosend the uplink subframe may comprise obtaining an identifier from theat least one of the plurality of uplink trigger subframes. As previouslynoted in the embodiments described above, this identifier may include arunning count of uplink trigger subframe, thereby replacing the need fora counter in the medical devices. Additionally or alternatively, theidentifier may comprise an address or other identifier of the medicaldevice.

As noted previously, some embodiments may provide for multiple uplinksubframes to be communicated between successive uplink triggersubframes. In some embodiments, the medical device may further identifythe at least one of the plurality of uplink trigger subframes byanalyzing a pair of pulses in the at least one of the plurality ofuplink trigger subframes, where the pair of pulses is modulated suchthat a first pulse has a first duration, and a second pulse has a secondration different than the first duration. As indicated in the embodimentillustrated in FIG. 4, different pulses may have different amplitudes,to help the medical device identify and parse pulse pairs. In someembodiments, analyzing the pair of pulses in the at least one of theplurality of uplink trigger subframes may comprise comparing a ratio ofthe first duration to the second duration. In some embodiments,determining when to send the uplink subframe may comprise adjusting alocal clock of the medical device based on when the least one of theplurality of uplink trigger subframes was received. For example, amedical device may synchronize its LO (or other clock based thereon)based on one an uplink trigger subframes received. Means for performingthe functionality at block 620 may comprise, for example, bus 905,processing unit(s) 910, memory 920, and/or other components of a medicalimplant 130, as illustrated in FIG. 9 and described in more detailbelow.

At block 630, the medical device sends the uplink subframe. Here, asnoted previously, the uplink subframe can be transmitted betweensuccessive uplink trigger subframes, each medical device transmittingits respective uplink subframe during its respective time slot duringthe communication frame. Means for performing the functionality at block630 may comprise, for example, bus 905, processing unit(s) 910, memory920, communication interface 930, antenna 935, and/or other componentsof a medical implant 130, as illustrated in FIG. 9 and described in moredetail below.

FIG. 7 is a flow diagram of an example method 700 of enablingsynchronize wireless communication with an interrogator device,according to an embodiment. The functionality described in one or bothblocks illustrated in FIG. 7 may be performed, for example, by aninterrogator device in a low-power wireless system, such as aninterrogator device 140 of the wireless medical implant systemillustrated in FIG. 1. Accordingly, means for performing thisfunctionality may include hardware and/or software components of aninterrogator device 140. An example of such hardware and/or softwarecomponents is illustrated in FIG. 8 and described in more detail below.

The functionality at block 710 comprises generating, with theinterrogator device, a communication frame comprising a synchronizationsubframe, a payload subframe, and a plurality of uplink triggersubframes. Again, the communication frame here may comprise an operatingframe as illustrated in FIG. 3, and described in more detail above. And,as noted above, plurality of uplink trigger subframes may be spacedapart in time such that at least one uplink subframe can be transmittedbetween successive uplink trigger subframes. In some embodiments, themethod may further comprise including, in each uplink trigger subframeof the plurality of uplink trigger subframes, and identifier unique therespective uplink trigger subframe. Again, this identifier may comprisean increasing uplink trigger subframe count, an address of a medicalimplant, and/or some other identifier to uniquely identify the uplinktrigger subframe. In some embodiments, the method may further compriseincluding, in each uplink trigger subframe of the plurality of uplinktrigger subframes, a first pair of pulses, where the first pair ofpulses uses a first modulation scheme such that a first pulse has afirst duration and a second pulse has a second ration different from thefirst duration. As noted above, the first modulation scheme may be basedon a particular ratio between long and short pulses. As such, accordingto some embodiments, the method may comprise causing a ratio of thefirst duration to the second ration to exceed a threshold. As notedabove, for example, a ratio of the long pulse to the short pulse mayexceed a threshold of 6:1, in some embodiments. (This threshold may belarger or smaller, depending on desired functionality. Of course, alower threshold (e.g., 1:6) may be exceeded if pulse durations arereversed.) In some embodiments, the first modulation scheme is differentthan a second modulation scheme used to modulate a second pair of pulsesin the payload subframe. For example, where the ratio of the duration oflong to short pulses in the first modulation scheme may meet or exceed6:1, the ratio of duration of long to short pulses in the secondmodulation scheme may be approximately or substantially 2:1. Of course,other undulation schemes may be used. Means for performing thefunctionality at block 710 may comprise, for example, bus 805,processing unit(s) 810, memory 850, and/or other components of aninterrogator device 140, as illustrated in FIG. 8 and described in moredetail below.

The functionality at block 720 comprises sending, with the interrogatordevice, the communication frame via an RF signal. As noted above, theinterrogator device may continue to transmit an RF carrier betweensuccessive uplink trigger subframes where medical devices (e.g., medicalimplants) may need the RF carrier for power purposes. Means forperforming the functionality at block 720 may comprise, for example, bus805, processing unit(s) 810, memory 850, communication interface 840,antenna 845, and/or other components of an interrogator device 140, asillustrated in FIG. 8 and described in more detail below.

The functionality at block 730 comprises receiving, at the interrogatordevice, at least one uplink subframe between successive uplink triggersubframes in the communication frame. As noted above, the interrogatordevice may continue to transmit an RF carrier between successive uplinktrigger subframes where medical devices (e.g., medical implants) mayneed the RF carrier for power purposes. Each medical device may transmitan uplink subframe during its allocated time slot (between uplinktrigger subframes). Means for performing the functionality at block 720may comprise, for example, bus 805, processing unit(s) 810, memory 850,communication interface 840, antenna 845, and/or other components of aninterrogator device 140, as illustrated in FIG. 8 and described in moredetail below.

FIG. 8 is a simplified block diagram of an interrogator device 140,according to an embodiment. The interrogator device 140 may comprise a“skin patch” (similar to the interrogator device of FIG. 1) or otherdevice configured to perform one or more of the functions of aninterrogator device as described in embodiments herein. FIG. 8 is meantonly to provide a generalized illustration of various components, any orall of which may be included or omitted as appropriate. The interrogatordevice 140 may be configured to execute one or more functions of themethods described herein, such as the method 700 illustrated in FIG. 7.It can be further noted that the interrogator device 140 may beconfigured to receive measurements from and/or stimulate a body partutilizing one or more medical devices, such as medical implants, withwhich the interrogator device 140 is in wireless communication, asdescribed in the embodiments above. In some embodiments, the particularmeasurements taken and/or stimulations may be determined by theinterrogator device 140 itself, and/or be determined by another device(such as a medical device, mobile phone, tablet, etc.) with which theinterrogator device 140 is in communication. A person of ordinary skillin the art will understand that, for the sake of simplicity, somecomponents (e.g., power source, clock, physical housing, etc.) are notshown.

The interrogator device 140 is shown comprising hardware elements thatcan be electrically coupled via a bus 805 (or may otherwise be incommunication, as appropriate). The hardware elements may include aprocessing unit(s) 810 which may comprise without limitation one or moregeneral-purpose processors, one or more special-purpose processors (suchas digital signal processing (DSP) chips, graphics accelerationprocessors, application specific integrated circuits (ASICs), and/or thelike), and/or other logic, processing structure, or means, which can beconfigured to perform one or more of the methods described herein.

Depending on desired functionality, the interrogator device 140 also maycomprise one or more input devices 820, which may comprise withoutlimitation one or more, touch sensors, buttons, switches, and/or moresophisticated input components, which may provide for user input, whichmay enable the system to power on, configure operation settings, and/orthe like. Output device(s) 830 may comprise, without limitation, lightemitting diode (LED)s, speakers, and/or more sophisticated outputcomponents, which may enable feedback to a user, such as an indicationthe implant system has been powered on, is in a particular state, isrunning low on power, and/or the like.

The interrogator device 140 might also include a communication interface840 and one or more antennas 845. This communication interface 840 andantenna(s) 845 can enable the interrogator device 140 to communicatewith and optionally power the medical implants of the wireless medicalimplant system. The one or more antennas 845 can be configured to, whenpowered properly, generate particular signals and/or fields tocommunicate with and/or power the medical implants, includingcommunicating medical implant selection methods as described herein. Aspreviously indicated, medical implants in some embodiments maycommunicate using RF backscatter, in which case the interrogator device140 may transmit an RF carrier signal, modulated by the medical implantsduring uplink communications.

In some embodiments, the processing unit(s) 810 and/or communicationinterface 840 (including software and/or firmware executed therewith)may create a communication frame and/or perform amplitude modulation ofan RF signal to encode the RF signal with the communication frame, asdescribed herein.

The communication interface 840 may further enable the interrogatordevice 140 to communicate with one or more devices outside thebiological measurement and stimulation system to which the interrogatordevice 140 belongs, such as a medical device, mobile phone, tablet, etc.In some embodiments, the one or more devices may execute a softwareapplication that provides a user interface (e.g., a graphical userinterface) for configuring and/or managing the operation of theinterrogator device 140.

The communication interface may include connectors and/or othercomponents for wired communications (e.g., universal serial bus (USB)Ethernet, optical, and/or other communication). Additionally, oralternatively, the communication interface 840 and optionally theantenna(s) 845 may be configured to provide wireless communications(e.g., via Bluetooth®, Bluetooth® low energy (BLE), Institute ofElectrical and Electronics Engineers (IEEE) 802.11, IEEE 802.15.4 (orZIGBEE®), Wi-Fi, WiMAX™, cellular communications, infrared, etc.). Assuch, the communication interface 840 may comprise without limitation amodem, a network card, an infrared communication device, a wirelesscommunication device, and/or a chipset.

The interrogator device 140 may further include and/or be incommunication with a memory 850. The memory 850 may comprise, withoutlimitation, local and/or network accessible storage such as optical,magnetic, solid-state storage (e.g., random access memory (“RAM”) and/ora read-only memory (“ROM”)), or any other non-transitory,computer-readable medium. The memory 850 may therefore make theinterrogator device 140 can be programmable, flash-updateable, and/orthe like. Such storage devices may be configured to implement anyappropriate data stores, including without limitation, various filesystems, database structures, and/or the like.

The memory 850 of the interrogator device 140 also can comprise softwareelements (not shown), including an operating system, device drivers,executable libraries, and/or other code, such as one or more applicationprograms, which may comprise computer programs provided by variousembodiments, and/or may be designed to implement methods, and/orconfigure systems, provided by other embodiments, as described herein.For example, one or more procedures described with respect to thefunctionality discussed above might be implemented as computer codeand/or instructions executable by the interrogator device 140 (and/orprocessing unit(s) 810 of the interrogator device 140). The memory 850may therefore comprise non-transitory machine-readable media having theinstructions and/or computer code embedded therein/thereon.

FIG. 9 is a simplified block diagram of a medical implant 130, accordingto an embodiment. The medical implant 130 may comprise a “neurograin”(similar to the medical implants 130 of FIG. 1) or other deviceconfigured to perform one or more of the functions of a medical implantof a biological measurement and stimulation system as described inembodiments herein. It will be understood that medical devices that arenot implanted or implantable, may comprise similar components. FIG. 9 ismeant only to provide a generalized illustration of various components,any or all of which may be included or omitted as appropriate. It can befurther noted that the medical implant 130 may be configured to takemeasurements and/or stimulate a body part as directed by an interrogatordevice 140 using communications such as those described in theembodiments herein. A person of ordinary skill in the art willunderstand that, for the sake of simplicity, some components (e.g.,power source, LO, physical housing, etc.) are not shown. It will beunderstood that, in most embodiments, hardware and/or softwareoptimizations may be made to help minimize power consumption.

The medical implant 130 is shown comprising hardware elements that canbe electrically coupled via a bus 905, or may otherwise be incommunication, as appropriate. The hardware elements may include aprocessing unit(s) 910 which may comprise without limitation one or moregeneral-purpose processors, one or more special-purpose processors(e.g., microprocessors), and/or other logic, processing structure, ormeans, which can be configured to perform one or more of the methodsdescribed herein. As a person of ordinary skill in the art willappreciate, the processing unit(s) 910, may further include one or moresplicers, counters, and/or other circuitry as described herein (and/ormay implement the functions of such circuitry and software) forprocessing incoming RF signal. Additionally or alternatively, suchcircuitry (or software) he be implemented in the communication interface930, described in more detail below.

The medical implant 130 may further include and/or be in communicationwith a memory 920. As with other components of the medical implant 130,the memory 920 may be optimized for minimum power consumption. In someembodiments, the memory 920 may be incorporated into the processingunit(s) 910. Depending on desired functionality, the memory (which caninclude a non-transitory computer-readable medium, such as a magnetic,optical, or solid-state medium) may include computer code and/orinstructions executable by the processing unit(s) 910 to perform one ormore functions described in the embodiments herein.

A communication interface 930 and antenna(s) 935 can enable the medicalimplant 130 to wirelessly communicate the interrogator device, asdescribed herein. The antenna(s) 935 may comprise a coiled or otherantenna configured to draw power from communications and/or othersignals or fields generated by the interrogator device, powering themedical implant 130. In some embodiments, the medical implant 130 mayfurther include an energy storage medium (e.g., a battery, capacitor,etc.) to store energy captured by the antenna(s) 935. In someembodiments, the communication interface 930 and antenna(s) 935 may beconfigured to the interrogator device using RF backscatter, as notedabove.

The stimulator(s) 940 of the medical implant 130 can enable the medicalimplant 130 to provide stimulation to a body part (e.g., biologicaltissue) in which the medical implant 130 is implanted. As such, thestimulator(s) 940 may comprise an electrode, LED, and/or other componentconfigured to provide electrical, optical, and/or other stimulation. Theprocessing unit(s) 910 may control the operation of the stimulator(s)940, and may therefore control the timing, amplitude, and/or otherstimulation provided by the stimulator(s) 940.

The sensor(s) 950 may comprise one or more sensors configured to receiveinput from a body part (e.g., biological tissue), in which the medicalimplant 130 is implanted. Sensors may therefore be configured to senseelectrical impulses, pressure, temperature, light,conductivity/resistivity, and/or other aspects of a body part. Asdescribed herein, embodiments may enable medical implant 130 to providethis information, via the communication interface 930, to aninterrogator. Depending on desired functionality, information receivedby the sensor(s) 950 may be encrypted, compressed, and/or otherwiseprocessed before it is transmitted via the communication interface 930.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized hardware might also be used, and/or particularelements might be implemented in hardware, software (including portablesoftware, such as applets, etc.), or both. Further, connection to othercomputing devices such as network input/output devices may be employed.

The methods, systems, and devices discussed herein are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, features described with respectto certain embodiments may be combined in various other embodiments.Different aspects and elements of the embodiments may be combined in asimilar manner. The various components of the figures provided hereincan be embodied in hardware and/or software. Also, technology evolvesand, thus, many of the elements are examples that do not limit the scopeof the disclosure to those specific examples.

It has proven convenient at times, principally for reasons of commonusage, to refer to such signals as bits, information, values, elements,symbols, characters, variables, terms, numbers, numerals, or the like.It should be understood, however, that all of these or similar terms areto be associated with appropriate physical quantities and are merelyconvenient labels. Unless specifically stated otherwise, as is apparentfrom the discussion above, it is appreciated that throughout thisSpecification discussions utilizing terms such as “processing,”“computing,” “calculating,” “determining,” “ascertaining,”“identifying,” “associating,” “measuring,” “performing,” or the likerefer to actions or processes of a specific apparatus, such as a specialpurpose computer or a similar special purpose electronic computingdevice. In the context of this Specification, therefore, a specialpurpose computer or a similar special purpose electronic computingdevice is capable of manipulating or transforming signals, typicallyrepresented as physical electronic, electrical, or magnetic quantitieswithin memories, registers, or other information storage devices,transmission devices, or display devices of the special purpose computeror similar special purpose electronic computing device.

Terms, “and” and “or” as used herein, may include a variety of meaningsthat also is expected to depend at least in part upon the context inwhich such terms are used. Typically, “or” if used to associate a list,such as A, B, or C, is intended to mean A, B, and C, here used in theinclusive sense, as well as A, B, or C, here used in the exclusivesense. In addition, the term “one or more” as used herein may be used todescribe any feature, structure, or characteristic in the singular ormay be used to describe some combination of features, structures, orcharacteristics. However, it should be noted that this is merely anillustrative example and claimed subject matter is not limited to thisexample. Furthermore, the term “at least one of” if used to associate alist, such as A, B, or C, can be interpreted to mean any combination ofA, B, and/or C, such as A, AB, AA, AAB, AABBCCC, etc.

Having described several embodiments, various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the disclosure. For example, the above elements may merely bea component of a larger system, wherein other rules may take precedenceover or otherwise modify the application of the invention. Also, anumber of steps may be undertaken before, during, or after the aboveelements are considered. Accordingly, the above description does notlimit the scope of the disclosure.

What is claimed is:
 1. A medical device comprising: a communication interface configured to receive a communication frame transmitted via a radio frequency (RF) signal and comprising: a synchronization subframe, a payload subframe, and a plurality of uplink trigger subframes spaced apart such that at least one uplink subframe can be transmitted between successive uplink trigger subframes; a processing unit communicatively coupled with the communication interface and configured to: determine when to send an uplink subframe via the communication interface based, at least in part, on when at least one of the plurality of uplink trigger subframes was received; and send, via the communication interface, the uplink subframe.
 2. The medical device of claim 1, wherein the processing unit is configured to determine when to send the uplink subframe by counting a number of uplink trigger subframes of the plurality of uplink trigger subframes.
 3. The medical device of claim 1, wherein the processing unit is configured to determine when to send the uplink subframe by obtaining an identifier from the at least one of the plurality of uplink trigger subframes.
 4. The medical device of claim 1, further comprising a local clock, wherein the processing unit is configured to determine when to send the uplink subframe by adjusting the local clock based on when the at least one of the plurality of uplink trigger subframes was received.
 5. The medical device of claim 1, wherein the processing unit is further configured to identify the at least one of the plurality of uplink trigger subframes by analyzing a pair of pulses in the at least one of the plurality of uplink trigger subframes, wherein the pair of pulses is modulated such that: a first pulse has a first duration, and a second pulse has a second duration different than the first duration.
 6. The medical device of claim 5, the processing unit is configured to analyze the pair of pulses in the at least one of the plurality of uplink trigger subframes by comparing a ratio of the first duration to the second duration.
 7. The medical device of claim 1, wherein the processing unit is configured to send the uplink subframe between successive uplink trigger subframes of the plurality of uplink trigger subframes.
 8. An interrogator device comprising: a processing unit configured to generate a communication frame comprising: a synchronization subframe, a payload subframe, and a plurality of uplink trigger subframes; a communication interface communicatively coupled with the processing unit and configured to: send the communication frame via a radio frequency (RF) signal; and receive at least one uplink frame between successive uplink trigger subframes in the communication frame.
 9. The interrogator device of claim 8, wherein the processing unit is further configured to include, in each uplink trigger subframe of the plurality of uplink trigger subframes, an identifier unique to the respective uplink trigger subframe.
 10. The interrogator device of claim 8, wherein the processing unit is further configured to include in each uplink trigger subframe of the plurality of uplink trigger subframe a first pair of pulses, wherein the first pair of pulses uses a first modulation scheme such that: a first pulse has a first duration, and a second pulse has a second duration different than the first duration.
 11. The interrogator device of claim 10, wherein the processing unit is further configured to cause a ratio of the first duration to the second duration to exceed a threshold.
 12. The interrogator device of claim 10, wherein the processing unit is configured to use a second modulation scheme to modulate a second pair of pulses in the payload subframe, wherein the second modulation scheme is different than the first modulation scheme.
 13. A method of synchronizing wireless communication at a medical device, the method comprising: receiving, at the medical device, a communication frame transmitted via a radio frequency (RF) signal and comprising: a synchronization subframe, a payload subframe, and a plurality of uplink trigger subframes spaced apart such that at least one uplink subframe can be transmitted between successive uplink trigger subframes; determining, with the medical device, when to send an uplink subframe based, at least in part, on when at least one of the plurality of uplink trigger subframes was received; and sending, with the medical device, the uplink subframe.
 14. The method of claim 13, wherein determining when to send the uplink subframe comprises counting a number of uplink trigger subframes of the plurality of uplink trigger subframes.
 15. The method of claim 13, wherein determining when to send the uplink subframe comprises obtaining an identifier from the at least one of the plurality of uplink trigger subframes.
 16. The method of claim 13, wherein determining when to send the uplink subframe comprises adjusting a local clock of the medical device based on when the at least one of the plurality of uplink trigger subframes was received.
 17. The method of claim 13, further comprising identifying the at least one of the plurality of uplink trigger subframes by analyzing a pair of pulses in the at least one of the plurality of uplink trigger subframes, wherein the pair of pulses is modulated such that: a first pulse has a first duration, and a second pulse has a second duration different than the first duration.
 18. The method of claim 17, wherein analyzing the pair of pulses in the at least one of the plurality of uplink trigger subframes comprises comparing a ratio of the first duration to the second duration.
 19. The method of claim 13, wherein sending the uplink subframe comprises sending the uplink subframe between successive uplink trigger subframes of the plurality of uplink trigger subframes.
 20. A method of enabling synchronized wireless communication with a interrogator device, the method comprising: generating, with the interrogator device, a communication frame comprising: a synchronization subframe, a payload subframe, and a plurality of uplink trigger subframes; sending, with the interrogator device, the communication frame via a radio frequency (RF) signal; and receiving, at the interrogator device, at least one uplink frame between successive uplink trigger subframes in the communication frame.
 21. The method of claim 20, further comprising including, in each uplink trigger subframe of the plurality of uplink trigger subframes, an identifier unique to the respective uplink trigger subframe.
 22. The method of claim 20, further comprising including in each uplink trigger subframe of the plurality of uplink trigger subframe a first pair of pulses, wherein the first pair of pulses uses a first modulation scheme such that: a first pulse has a first duration, and a second pulse has a second duration different than the first duration.
 23. The method of claim 22, further comprising causing a ratio of the first duration to the second duration to exceed a threshold.
 24. The method of claim 22, wherein the first modulation scheme is different than a second modulation scheme used to modulate a second pair of pulses in the payload subframe. 